In recent years, it has been a wide practice that a transparent film on which a toner image had been formed by electrophotography is applied to an OHP (overhead projector) to project the image.
In an electrophotographic system, discharging is conducted, for example, charging a transfer film at a high voltage when a toner on an electrostatic latent image is transferred to the transfer film. Therefore, if the surface resistance of the film is high, a discharging mark occurs on the film in the copying machine. In order to present the discharging mark, it has been proposed that the surface resistance of the transfer film be controlled to a predetermined range.
However, a full-color image system wherein four color toners are transferred has a problem that if the surface resistance of the transfer film is too low, a toner which has been early transferred leaks charge onto the transfer film, reducing its adhesion to the transfer film, and is then transferred back to the electrostatic latent image retaining unit upon the subsequent toner transfer, causing color loss or deterioration of image quality and density.
Further, in the conventional electrophotography, since the interface of the toner with air gives a great refraction of incident light beam, only a dark image for projection with a low saturation can be obtained.
In order to overcome these difficulties, it is necessary that the roughness of the toner image formed on the transparent film be minimized. Many approaches have been heretofore proposed as discussed below. However, these approaches are not perfect solutions to the above mentioned problems.
JP-B-51-34734 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses that the optimum surface resistivity of a black-and-white electrophotographic transfer film is in the range of 10.sup.10 to 10.sup.12 .OMEGA.. However, in a full-color system, if the surface resistivity is simply lowered, retransfer will occur at 10.sup.11 .OMEGA.. Furthermore, if the optimum surface resistivity is limited to 10.sup.12 .OMEGA., the range of the optimum surface resistivity is extremely limited in JP-B-51-34734 and a full-color system.
JP-A-59-184361 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes an approach in which a lacquer is spray-coated onto the surface of a toner image. However, this approach is disadvantageous in that the solvent for lacquer dissolves the toner, causing a drop in the image sharpness, unevenness of color or stain on non-image portion.
JP-A-60-52861 proposes an approach in which a toner image is covered with a laminated film. JP-A-61-36756 and 61-36762 propose an approach which comprises laminating a transparent film on a toner image, fixing the lamination through heat rolls, and then peeling the transparent film.
However, these approaches are disadvantageous in that they require a large number of processing steps following the image formation or cause destruction of the toner image upon peeling of the transparent film.
JP-A-63-80273 proposes an approach which comprises fixing at a temperature high enough to dissolve a toner, an approach which comprises fixing with a solvent such as toluene, an approach which comprises polishing the surface of a fixed image, and an approach which comprises coating the fixed toner image with a transparent coating which does not dissolve the toner.
However, in the case of roller fixing at an elevated temperature, when it is intended to minimize the roughness of a portion with less toner such as halftone portion, an offset will occur at a high density portion with much toner. In the case using a non-contact heat fixing apparatus such as oven, surge will occur in the transparent film, and a considerable period of fixing time is required to obtain an image having a sufficient transmission.
In the case of the approach with solvent fixing, when the fluidity of the toner is increased to an extent such that the toner roughness of the halftone portion is decreased, image collapse or running will occur at a high density portion.
In the case of the approach which comprises polishing the surface of an image, although the transmission can be increased at a portion with a relatively large amount of a toner, the toner roughness cannot be sufficiently reduced at a low density portion.
In the case of the approach which comprises coating the toner image with a transparent coating which does not dissolve the toner, a definite interface may occur on the toner grains and toner image, disadvantageously scattering the incident light beam and thus giving a dark image for projection with a low saturation.
JP-A-2-263642 proposes an approach for minimizing the roughness of a toner image formed on a transparent film which comprises providing, on the transparent film, a transparent resin layer compatible with a toner binding resin and having a greater storage elastic modulus than the toner binding resin at the toner fixing temperature. However, in this approach, since the storage elastic modulus of the transparent resin layer is greater than that of the toner binding resin, the toner can hardly sink in the transparent resin layer or hardly diffuse horizontally. In order to minimize the roughness of the toner image, it is necessary that the transparent resin layer be sufficiently thick and the toner be forced to sink in the transparent resin layer under pressure by a heat roller having a high hardness. The increase in the thickness of the transparent resin layer causes image blurring or distortion or image cracking by bending. The forced fixing by a heat roller having a high hardness causes wrinkling upon fixing, curling after fixing, etc.
Moreover, in the digital color image formation system, various color densities are given by changing the area of fine dots or lines. Therefore, the surface of a toner forming an image can be sufficiently smoothed at a high density portion by a heat roll fixing apparatus, but the roughness of the halftone portion due to the configuration of dot or line image is too large to smooth sufficiently. Thus, the refraction of the incident light beam gives a dark image with a low saturation at a halftone portion in the image for projection. When a natural image such as photograph is reproduced, a shadow which is not present in the original (hereinafer referred to as "pseudo-outline") is formed at the halftone portion.
In order to eliminate pseudo-outline which often occurs in the digital color image formation system, it is necessary that a toner image be allowed to sufficiently sink in the transparent resin layer on the transparent film or to diffuse horizontally. In the case of the approach as proposed in the above cited JP-A-2-263642, which comprises providing on the transparent film a transparent resin having a greater storage elastic modulus than the toner binding resin, the toner image can hardly sink in the transparent resin layer or hardly diffuse horizontally in the transparent resin layer, making it impossible to sufficiently reduce the roughness of the toner image at the halftone portion. Therefore, this approach can hardly inhibit the generation of pseudo-outline.